Although applicable to any desired stiffening elements, the present invention as well as the problem on which it is based will be explained in more detail with reference to frames for stiffening a fuselage shell of an aircraft.
Fuselage shells for aircraft are normally produced using a so-called lightweight structure from an outer skin, which is reinforced on the inside by a two-dimensional structure composed of stringers, which run in the longitudinal direction of the aircraft, and frames which run in the lateral direction as stiffening elements. In this case, different profile shapes are known for the stringers and are equivalent, for example, to an “L”, an “Ω” or an inverted “T”. In this case a part of the profile respectively forms a foot section of the stringer, which rests flat on the inside of the outer skin and is firmly connected to it by various techniques such as riveting, welding or adhesive bonding.
In order to allow frames which run in the lateral direction for further stiffening to be fitted into the substructure which is formed by the connection of the skin and stringers, cutouts are normally provided on the frames at the crossing areas of the frames and stringers, so that the stringers are passed through the cutouts underneath the frames in the stiffening structure which is formed together with the frames. The connection between the frame and the outer skin in the areas between the cutouts is in this case produced either by means of a separate clip, for example a molded sheet-metal part, or, in the case of a so-called integral frame, by a foot section which is integrated in the frame.
In order to achieve good mechanical robustness, it is desirable for the clip or the foot section of the frame to be passed not only over the outer skin itself but also over foot sections of the stringers, for example over a foot section of an Ω-shaped or T-shaped stringer. Since the junction between the outer skin and the foot section of the stringer represents a step, the clip or foot section of the frame must be appropriately matched to the shape of this step, in order to fix the frame both in the area of the foot sections of the stringers and in the area of the outer skin which is exposed between the stringers.
For example, in the case of a clip formed from a molded sheet-metal part it is possible to provide apertures, corresponding to the shape of the step, in the molded sheet-metal part during production, or to introduce these by subsequent shaping. In the case of an integral frame which is manufactured from aluminum or from some other metal, the foot section can be adapted by milling the shape of the step-like substructure.
Fuselage shells, stringers and frames are, however, increasingly produced from fiber composite materials, in particular from carbon-fiber-reinforced plastic (CFRP), for weight-saving reasons. In this case, they have a structure composed of a plurality of layers of fiber mats, which are connected to one another by means of a cured resin matrix, in particular epoxy resin. This results in the problem that step-like milling of a foot section of an integral frame manufactured from CFRP is impossible in the manner described for an aluminum frame, since CFRP is subject to splitting and delamination if milled in a direction at a sharp angle to the fiber profile.
One method that is currently used for CFRP integral frames therefore provides for shims to be inserted between the foot section of the frame and the deeper areas of the step-like substructure in order to compensate for the steps in the substructure. This method has the disadvantage that numerous additional parts which fit accurately must be manufactured with the shims, thus increasing production costs and the total weight of the structure.
A further problem which occurs when using integral frames manufactured from CFRP is that the curing process during the production of the CFRP frames means that it is not possible to achieve the same dimensional accuracy as in the case of milled aluminum frames. Shims are also used for this reason, in the stated manner and with the stated disadvantages.